Calcium iron phosphate catalyst and method for dehydrogenating and cracking alkanes and olefins



3,327,009 CALCIUM IRON PHOSPHATE CATALYST AND METHOD FGR DEHYDROGENATINGAND CRACKING ALKANES AND OLEFLNS Charles R. Noddings, Midland, andRonald G. Gates, Breckenridge, Mich, assignors to The Dow ChemicalCompany, Midland, Mich, a corporation of Delaware Filed Jan. 6, 1964-,Ser. No. 335,813 4 Claims. (Cl. 260-680) This invention concerns a newcatalyst and a process employing the catalyst for the dehydrogenationand cracking of aliphatic hydrocarbons, primarily parafiins and olefins,having three or more carbon atoms in the molecule. It pertainsespecially to a novel catalyst and a process employing the catalyst forthe dehydrogenation and cracking of parafiins and olefins having threeor more and preferably four carbon atoms in the carbon chain to form bydehydrogenation and/or carbon bond splitting the correspondingunsaturated hydrocarbon, e.g., olefins and dlenes as well as the lowercarbon chain compounds such as two and three carbon chain compounds, forexample propylene, propane, ethylene, ethane, butene and butadiene when4 or more carbon atoms are employed, along with dehydrogenated carbonchain compounds having the same number of carbon atoms as the startinghydrocarbon.

It is, of course, well known that aliphatic hydrocarbons, e.g.,petroleum fractions (mixed hydrocarbons) or individual parafiins orolefins, can be pyrolyzed to obtain a mixture of products comprising asmall, though appreciable, proportion of conjugated diolefins and alarger proportion of shorter chain length unsaturate products. Duringsuch pyrolysis, it is thus evident that several different kinds ofreactions usually occur simultaneously, e.g., dehydrogenation, crackingof the carbon-to-carbon linkages in the molecule to form productscontaining fewer carbon atoms per molecule than the originalhydrocarbon, and polymerization of unsaturated hydrocarbons, so that theproduct is, in most instances, a hydrocarbon mixture. An example of suchproduct is cracked-oil gas, containing parafiinic hydrocarbons rangingin chain length from methane to hexane, olefins ranging from ethylene tohexylene, and a small amount, usually less than of less saturatedhydrocarbons such as butadiene, isoprene, piperylene and acetylenichydrocarbons. The difficulties involved in recovering the more usefulproducts from such mixture add greatly to their cost.

It is an object of this invention to provide an improved method for thedehydrogenation and/or cracking of C and higher hydrocarbons, andparticularly C and higher hydrocarbons. Another object of the presentinvention is to provide such a method whereby useful organic products(that is, products other than CO carbon and hydrogen) from the pyrolysis(i.e., cracking and/or dehydrogenation) are obtained in quantities whichincrease the economical value of the products over that of the startingcarbon compounds. A further object is to provide a set of operatingconditions under which the new catalyst may effectively be used for theforegoing purposes. Other objects will be apparent from the followingdescription of the invention.

We have found that a calcium-iron phosphate containing an average ofbetween 6 and 12 and preferably from about 6 to about 9 atoms of calciumper atom of iron is, under certain operating conditions, effective incatalyzing the thermal dehydrogenation and/ or cracking of Chydrocarbons, particularly C and higher hydrocarbons, to C and Chydrocarbons, containing a high proportion of olefins.

The catalyst is prepared by mixing together watersoluble metal salts ofcalcium and iron with a water- T ates atent O F 3,327,009 Patented June20, 1967 ICC soluble form of the ortho-phosphate moiety (POE) in anaqueous medium under conditions such that the pH is within the range offrom 2 to 10. Material of good catalytic quality is obtained when thecalcium and iron are employed to provide from 6 to 12 moles of calciumper mole of iron. Further, while not critical, but desirable, thephosphate moiety is employed in a slight excess over that which istheoretically necessary to combine with the metal ions to form a metalortho-phosphate. It is to be understood that the pH may, but does nothave to, be maintained Within the operative range during mixing but canbe adjusted, after mixing, by addition of a 'base or acid as necessaryto the reaction mixture to bring the solution within the desired rangethereby causing precipitation of a catalytic material.

The contacting and mixing of the reactants in accordance with the aboverecitation can be carried out in several manners, such assimultaneously, stepwise or intermittently, each in either batchwise orcontinuous manner.

Examples of salts which may be used as starting materials in preparingthe catalyst are the chlorides, bromides, nitrates, and acetates, etc.,of calcium and iron. Examples of soluble phosphates that may be employedas starting materials are disodium phosphate, trisodium phosphate,dipotassium phosphate, di-ammonium phosphate, etc.

The catalyst can also be prepared in either a batchwise manner or acontinuous manner by feeding separate streams of an alkali, preferablyaqueous ammonia, although other bases can be employed as well asmixtures of two different bases, and as either a single or as a separatestream an aqueous solution of calcium and iron salts (in relativeproportions corresponding to between 6 and 12 and preferably about 6 to9 atoms of calcium per atom of iron), and either a separate or as a partof any one of the aforesaid streams a dissolved orthophosphate, into areaction chamber. The relative rates of flow being adjusted such thatthe resultant mixture will achieve continuously or upon completion ofthe mixing a pH between 2 and 10. It is desirable in a continuous orstepwise operation to retain within the reaction zone a portion of thecalcium-iron phosphate which forms and precipitates. This isconveniently achieved by adjusting the outflow of the calcium-ironphosphate precipitated to retain a portion of the flocculent materialwhich settles rapidly to form, as a lower layer of the resultingmixture, an aqueous calcium-iron phosphate slurry that contains 2% byweight or more, usually from 7.5 to 10%, of the calcium-iron phosphate.The reaction mixture, or preferably the settled lower layer thereof, maybe filtered to obtain a filter cake which contains 10% or more, usuallyabout 22%, of the calcium-iron phosphate.

In order to obtain a rapid settling calcium-iron phosphate of goodcatalytic activity, it is important that the two or more streams of theabove-mentioned starting materials flow into admixture with one another,e.g., within a body of the resulting mixture, at relative rates such asto maintain the resultant mixture at a pH value between 2 and 10. Thephosphate precipitated from a mixture of higher pH value which is thenadjusted to about 2 to 10 is of good catalytic activity, but isextremely slow in settling. The phosphate precipitated from a mixture ofpH value below 2 settles rapidly, but is less active as a catalyst forthe cracking and dehydrogenation of C hydrocarbons than is phosphateprecipitated from mixtures within the range of 2 to 10 pH values. Italso appears necessary, in order to obtain a calcium-iron phosphateproduct of rapid settling rate, that a portion of the precipitatedphosphate be retained in the mixing and reaction zone so that, once theprocess is started, the catalyst is being formed and precipitated in thepresence of a slurry of the calcium-iron phosphate. It is probable thatthe catalyst already precipitated serves as nuclei for precipitation offurther amounts of catalytic material and aids in controlling theparticle size and physical form of thematerial undergoing precipitation,but the invention is not restricted to this theory as to a reason forthe result obtained. Presence of preformed particles of catalyst duringprecipitation of further amounts of the latter is not, of itself,sufficient to cause formation of a rapid settling product, i.e., it isalso necessary that the reaction mixture as a whole be maintained at anaverage pH value of between 2 and 10.

The procedure in bringing the two streamsof starting materials togetherand admixing them also has an influence on the rate of settling of thecatalyst which is precipitated. It is desirable that the points of feed,to the aqueous mixture in the mixing chamber of the streams of'the twostarting materials, be remote from one another and that the mixture bestirred, or otherwise agitated, during introduction of the startingmaterials. Usually, inlets for the different kinds of starting materialsare separated by a distance of a foot or more. Either starting materialmay, of course, be introduced through a plurality of inlets. It isprobable that these precautions of separating the points of feed of thedifferent starting materials and of agitating the mixture result inactual contact between the starting materials in a zone, or zones, ofapproximately the pH value which is average for the mixture as a whole,i.e., the procedure just recommended presumably results in formation andcoagulation of calcium-iron phosphate in zones which are actually at pHvalues between 2 and 10. It will be understood that the minimum distancebetween points of feed of the different starting materials is dependentin part upon the rates of feed, and that it may be less with low ratesof feed than with high rates of feed. An increase in the degree, orefficiency, of stirring of the mixture will also permit a decrease inminimum distance between the points of feed. In actual manufacture ofthe phosphate, the points of feed of starting materials mayadvantageously be separated by a distance of feet or more.

Usually water is employed as the solvent for the starting materials, butother ionizing solvents, e.g., aqueous alcohol, may in some instances beused.

In any event after the reaction is complete, the precipitate isseparated from the liquor by filtration or decantation and is washedwith water, decanting or filtering after each washing. The washingshould be carried out so as to remove as thoroughly as possible readilysoluble compounds from the product, since such impurities have adisturbing and erratic action on the thermal decomposition ofhydrocarbons. Of particular attention are the unreacted chlorides orbyproduct chlorides which, if retained in the catalyst, tend todeactivate the latter. The catalyst is, at this stage in itspreparation, a solid or gel-like substance which is apparentlyamorphous.

-After being Washed with water, the product is dried, usually attemperatures between 60 and 150 C. The dried product is a hard gelusually of yellowish color. The gel may be crushed or otherwise reducedto granules, or small lumps, and be used directly as a dehydrogenationcatalyst. However, it is preferably pulverized, e.g., to a particle sizecapable of passing a ZS-mesh screen, and the powdered product is treatedwith a lubricant and is pressed into the form of pills, tablets, orgranules of size suitable for use as a catalyst, e.g., into the form oftablets of from to A2 inch diameter. The lubricant serves to lubricatethe particles during the operation of pressing them into pills and itsuse permits the formation of pills of greater strength and durabilitythan are otherwise obtained. As the lubricant, we preferably use asubstance capable of being removed by vaporization or oxidation from theproduct, e.g., a substance such as graphite, a vegetable oil, or ahydrocarbon oil, etc.

C and higher hydrocarbons can be cracked and de- 4 hydrogenated in thepresence of steam and the catalyst of the present invention attemperatures between 600 and 750 C., and in some instances attemperatures as much as 50 C. below or above this range. The reaction isadvantageously carried out at temperatures between 650 and 700 C.

Except for the foregoing limitations, the conditions under which thedehydrogenation reaction is carried out may be varied widely. Also, themethod is operable at atmospheric, subatmospheric, or atsuperatmospheric pressures, provided the hydrocarbon reactant is invaporized form. In some instances, the yield of dehydrogenated productdecreases upon increase of the reaction pressure above atmospheric.However, the ability to operate at an increased pressure is ofconsiderable advantage, since condensation of the reaction products maythereby be facilitated. In general, the proportion of hydrocarbonreacted and also the amount of byproduct formation per pass through thecatalyst bed tend to decrease with increase in the rate of vapor flow,and vice versa.

In producing cracked and dehydrogenated hydrocarbon products inaccordance with the invention, a reaction chamber is charged with thegranular catalyst and the lubricant employed is removed from thecatalyst. This is usually accomplished by passing an O -containing gassuch as oxygen or air, preferably a mixture of about equal volumes ofair and steam, through the catalyst bed at a high temperature, e.g., 450to 750 C. When the lubricant used in preparing the catalyst granules isa substance capable of being vaporized, e.g., a mineral or vegetableoil, the step of treating the catalyst with air may be preceded by oneof passing an inert gas or vapor such as steam, nitrogen, or carbondioxide over the catalyst so as to vaporize at least a portion of thebinding agent from the catalyst granules.

After freeing the catalyst of the lubricant, the catalyst bed is sweptfree of the 0 or air with steam and is heated to the desired reactiontemperature, preferably by passing superheated steam through the same. Amixture of steam and the hydrocarbon reactant, e.g., propene, propane,butylene, amylene, hexylene, butane, pentane, or hexane, having at leastthree carbon atoms, is then passed through the catalyst bed at atemperature between 600 and 750 C., and preferably between 650 and 700C. The usual procedure is to pass the hydrocarbon gas into admixturewith steam which has been superheated to 750 C. or above, i.e., to atemperature sufiicient so that the resultant mixture is at the desiredreaction temperature, and to pass the mixture through the bed ofcatalyst. However, the heat may be supplied in other ways, e.g., byforming the steam and hydrocarbon mixture at a lower temperature andpassing the mixture through a preheater to bring it to the desiredtemperature, or by externally heating the catalyst chamber itself. Theyield of olefins is usually highest when from 10 to 20 volumes of steamare employed per volume of the gaseous or vaporized hydrocarbon, but thesteam may be used in smaller or larger proportions if desired. Ashereinbefore mentioned, the rate of vapor flow through the catalystchamber may be varied widely, but in practice the flow usuallycorresponds to between and 700 liters of the hydrocarbon (expressed asat 0 C. and 760 millimeters pressure) per liter of catalyst bed perhour.

The vapors issuing from the catalyst chamber are ordinarily passedthrough heat exchangers and other cooling devices to condense first thewater and then the hydrocarbon products. By repeatedly recycling theunreacted hydrocarbons, an olefin product may be produced in a 60% yieldor higher and usually in a yield of from 70 to 75% of theoretical orhigher.

During use in the process, the catalyst gradually accumulates a smallamount of carbon, or non-volatile organic material, and loses itsactivity. Accordingly, flow of the hydrocarbon starting material isperiodically interrupted and air, admixed with the steam, is blownthrough the catalyst bed, e.g., at temperatures between 450 and 700 C.,and preferably at the dehydrogenating temperature, to oxidize and removethe carbonaceous or organic material and thus reactivate the catalyst.Usually from to 30 minutes is required to carry out this reactivationstep. However, if, during compounding of the catalyst into tablet form,an agent having the property of catalyzing the oxidation of carbon isadmixed therewith, the time subsequently required for reactivating thecatalyst with steam and air may be reduced markedly. For instance, theincorporation of one or two percent by weight of chromic oxide in thecatalyst tablets facilitates reactivation of the catalyst. Other agentshaving the property of catalyzing the burning of carbon are known to theart.

After completing the reactivation step, the catalyst chamber is againswept free of air with steam and the introduction of hydrocarbons,together with the steam, is resumed. Usually, reactivation of a catalystis advisable after from to 60 minutes of use in the dehydrogenationreaction. In practice, two or more catalyst chambers are preferablyemployed in a system provided with connections for passing the reactionmixture alternately through diiferent catalysts beds. One catalyst bedis usually employed in the dehydrogenation reaction while another isbeing reactivated. By operating in this manner, the dehydrogenationreaction may be carried out continuously.

The following example illustrates the present invention, but is not tobe construed as limiting:

Example In the manner shown in the accompanying drawing, 18.7 gram molesof calcium chloride as a 36.5 weight percent aqueous solution thereofwas mixed in a vessel with 3.12 gram moles of iron chloride as a 38.5weight percent aqueous solution and 19.2 gram moles of phosphoric acidas a 75 weight percent aqueous solution and the resulting mixture isdiluted with water to a total volume of 82 gallons. Upon completion ofthe addition of the above enumerated chemicals to the vessel reactor, anaqueous 13.1 weight percent ammonium hydroxide solution was, or hadbeen, added. In some instances, the aqueous ammonium hydroxide was addedtogether with the reactants, in others after addition of all of thereactants, and in still others the phosphoric acid and ammonia werefirst mixed and then admixed with the other reactants. The reaction masswas continuously stirred and base or acid added to produce and maintaina pH of the system between 2 and 10. In the specific instance 48.2 grammoles of ammonium hydroxide were required to maintain the pH at 8.8 atthe end of 2.5 hours of reaction. The reaction was considered completewhen the final pH remained constant. Thereafter the reaction mass wasallowed to settle overnight after which the supernatant liquid above theprecipitate was drawn 01f (approx. 14 gallons decanted) and theresulting thick slurry filtered and washed with water. The filtrate wasdiscarded. In the specific instance the slurry was washed with water 11times until chloride free, then removed and dried at 100 C. in a rotarydrier. The dry powder was recovered to the extent of 98% of thetheoretical yield, based on the starting materials used, and wascrushed, mixed with 2% by weight of a lubricant grade graphite andexpressed into pellets about inch in diameter and inch long. Thegraphite was burned off by treating the pellets with air and steam atabout 650 C. for about 6 hours. The resulting catalyst pellets weretested as cracking catalyst at 700 C., 150 v./v. hr. (voltime of gas perunit volume of catalyst per hour) (S.T.P.) with 99% n-butane, 3000 v./v.hr. of steam and 1.0 hr. cycle, half of which was regenerationaccomplished by passing v./v. hr. of air and 3000 v./v. hr. of steam at700 C. through the bed. Of the butane fed to the reactor, there wasobtained a yield 70.5% of C 11 (36.5%) and C H (34%), plus an 8.5% yieldof C 11 and a 4.5% yield of C H all based on the carbon content of theamount of butane converted, which conversion was 20%.

We claim:

1. The method which comprises dehydrogenating and cracking an aliphatichydrocarbon having at least 3 carbon atoms by passing the hydrocarbontogether with steam at a temperature between 600 and 750 C. in contactwith a catalyst composed of a metal phosphate material consistingessentially of phosphate radicals chemically combined with calcium andiron in the relative proportions of between 6 and 12 atoms of calciumper atom of iron which metal phosphate material is preparable by mixinga solution of soluble salts of calcium and iron with a solution of asoluble orthophosphate and precipitating in the gel form said metalphosphate material from the mixture at a pH of between about 2 to 10.

2. The method of claim 1 which comprises passing hydrocarbon vaporscontaining a paraffin having 4 carbon atoms and between 10 and 20volumes of steam into contact with the catalyst.

3.. The method of claim 2 wherein said paraffin is butane.

4. A metal phosphate material prepared by co-precipitation in the gelform from a solution of pH about 2 to about 10 of a water-solublecalcium salt, a water-soluble iron salt, and a phosphate, which metalphosphate material contains between about 6 and 12 atoms of calcium peratom of iron.

References Cited UNITED STATES PATENTS 2,542,813 2/1951 Heath 2524372,816,081 12/1957 Heath et a1 252437 3,149,082 9/1964 Bowman et al252437 3,188,350 6/1965 Martin et al 252437 FOREIGN PATENTS 1,279,57811/1961 France.

DELBERT E. GANTZ, Primary Examiner.

R. H. SHUBERT, Assistant Examiner.

1. THE METHOD WHICH COMPRISES DEHYDROGENATING AND CRACKING AN ALIPHATIC HYDROCARBON HAVING AT LEAST 3 CARBON ATOMS BY PASSING THE HYDROCARBON TOGETHER WITH STEAM AT A TEMPERATURE BETWEEN 600* AND 750*C. IN CONTACT WITH A CATALYST COMPOSED OF A METAL PHOSPHATE MATERIAL CONSISTING ESSENTIALLY OF PHOSPHATE RADICALS CHEMICALLY COMBINED WITH CALCIUM AND IRON IN THE RELATIVE PROPORTIONS OF BETWEEN 6 AND 12 ATOMS OF CALCIUM PER ATOM OF IRON WHICH METAL PHOSPHATE MATERIAL IS PREPARABLE BY MIXING A SOLUTION OF SOLUBLE SALTS OF CALCIUM AND IRON WITH A SOLUTION OF A SOLUBLE ORTHOPHOSPHATE AND PRECIPITATING IN THE GEL FORM SAID METAL PHOSPHATE MATERIAL FROM THE MIXTURE AT A PH OF BETWEEN ABOUT 2 TO
 10. 